Ln Dependence of the Large-Capacity Oxygen Storage/Release Property of Ln Oxysulfate/Oxysulfide Systems

Abstract

In a series of large-capacity oxygen storage materials using the redox between Ln 2 O 2 SO 4 (S 6+ ) and Ln 2 O 2 S(S 2-), the Pr system can work at as low as 600 °C, compared to g650 °C required for Ln ) La, Nd, and Sm. The exceptional character of the Pr system has been studied from physicochemical points of view by means of thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray diffraction, Rietveld analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Unlike the other Ln oxysulfates/oxysulfides, the Pr system contained a considerable amount of tetravalent cation (Pr 4+ ) on the surface. The smooth redox between Pr 3+ and Pr 4+ would promote the oxidation of bulk Pr 2 O 2 S to Pr 2 O 2 SO 4 . On the other hand, the smooth reduction of Pr 2 O 2 SO 4 to Pr 2 O 2 S appears to be associated with a local structure of sulfate. The X-ray and spectroscopic analysis predicted that the tetrahedral SO 4 unit of Pr 2 O 2 SO 4 is more distorted than that of La 2 O 2 SO 4 . Instability caused by the stronger distortion of SO 4 would lead to the easier reduction to S 2species. A synergy of these two different effects on the redox process of the Pr system seems to be a possible reason for the successful oxygen storage/release cycles at lowest possible temperatures.

X-ray structural study on the different redox behavior of La and Pr oxysulfates/oxysulfides

“…The La 2 O 2 2+ layer in both structures consists of OLn 4 tetrahedra linked together by sharing of edges [6,7]. Although the structural parameters of the Pr 2 O 2 SO 4 were recently reported in our recent paper [8], those of Pr 2 O 2 S have not been reported so far. The diffraction patterns suggested La and Pr compounds would be isostructural.…”

“…1) is not consistent with the larger distortion in Pr 2 O 2 SO 4 . Higher rate of reoxidation of Pr 2 O 2 S may be explained by the smooth redox between Pr 3+ and Pr 4+ , which affects the reoxidation process [8]. The creation of Pr 4+ could actually be detected by XPS on the surface of Pr 2 O 2 S and Pr 2 O 2 SO 4 .…”

Electronic Structure of Ce-doped ZrO2Film: Study of DFT Calculation and Photoelectron Spectroscopy

“…Doping of lanthanides to ZrO 2 stabilize oxygen deficiency [13]. Therefore, doping of Ce increase formation energy for oxygen intestinal, which well explains the change in chemical state.…”

A new continuous two-step molecular precursor route to rare-earth oxysulfides Ln2O2S

“…800 1C [52][53][54], we designed a two-step continuous process in order to synthesize the rare-earth oxysulfide. Rather than trying to achieve direct lanthanide oxysulfide synthesis by a one-step molecular precursor combustion process, this work aimed for the readily obtained oxysulfate phase beforehand, this phase being easily reduced afterwards by using diluted H 2 atmosphere (H 2 (5%)-Ar(95%)) at moderate temperature.…”